Local pro-Ukrainian activists said large numbers of Russian soldiers and armoured vehicles were in the area.

“The occupiers are conducting manoeuvres and we should understand that at any minute, at any hour, they could start a large-scale or small-scale attack,” said Andriy Lysenko, a military spokesman for Ukraine’s presidential administration.

On Saturday, a bomb exploded in the separatist-held city of Luhansk beside an SUV carrying the local militant leader, Igor Plotnitsky.

The device appeared to be attached to a lamppost, which was split by the blast.

The extent of Plotnitsky’s injuries was not clear, but separatist-run media released what they said was a recording made by him as he recovered in hospital, during which he accused Ukraine and its US backers of being behind the attack.